MSFT follows the N+2 model for servicing Windows versions for security and o/s system updates – i.e. only the two most current versions declared as Current Branch For Business(CBB) are supported

The last update(security and o/s) for Windows 10 Version 1507 will be March 2017

At the end of March 2017 only Windows 10 versions 1511 and 1607 will be serviced and supported. Eventually, the next version of Windows(Creators Update) will be declared CBB and sometime after that declaration Windows 10 Version 1511 will be given an end-of-support date for servicing and support with Windows updates.

This ‘N+2’ model explains a piece of the Windows 10 ‘Windows-as-a-Service’ model with respect to the version of Windows 10 installed on a given device

‘Supported’ does not necessarily mean the useful life of the device. ‘Supported’ from a service standpoint is the support cycle for a version of Windows 10 on a device for the supported lifetime of the device – i.e. the device must be capable(assuming the hardware can handle it) of upgrading to a version within the confines of the N+2 model.

e.g. Windows 10 will still work on a device with Windows 10’s initial version 1507 installed(released July 2015), but in the case of 1507 no updates will be available after March 2017. And in time, Windows 10 second major release 1511 will follow the same pattern(end of support) in the future after the next Win10 major version(The Creators Update) is released next spring.

Windows 10 1507 was the version released and available under the terms of the free-Windows 10 upgrade offer which just recently ended on July 29, 2016.

The ‘free-upgrade’ offer from a support standpoint was often misrepresented and misunderstood with many falsely referring or stating support for Windows 10 1507 as being applicable to the the ‘life of the device on which the free Windows 10’ was installed. The free-upgrade offer version(1507) still falls under the N+2 servicing model – no upgrades for the version after March 2017.

Thus, to retain the privilege of being supported a device with the Windows 10 1507 free-upgrade version(as well as purchased or OEM installed devices with Windows 10 1507) is required to upgrade to a version of Windows 10 within the confines of the ‘N+2’ servicing model – thus after March 2017, only Windows 10 1511 and 1607 will be the supported operating systems, and likewise, as noted above, once the next version is released(Creator’s Update) Windows 10 1511 will eventually fall off the ‘Windows-as-a-Service’ support lifecycle.

As we described in the Windows as a service docs at http://aka.ms/waas, we service and support two CBB releases at all times, so the end of servicing for a particular release is dependent on the timing of the N+2 release. It will be at least 18 months, but can be longer when releases are more than six months apart.

Because there are two newer CBB releases, Windows 10 1507 will not be serviced much longer – as I mentioned in a previous reply, the 60-day countdown (the grace period described in the Windows as a service docs) will start in January, meaning the last updates for Windows 10 1507 will be in March.

For Windows 10 1511, the end of servicing will be about six months after the CBB declaration for the next Windows 10 feature update (the Creators Update, due out early next year). So Windows 10 1511 will be serviced at least through the middle of 2017, possibly longer depending on the actual release date of the next feature update.